A) Field of the Invention
This invention relates to a liquid crystal display element.
B) Description of the Related Art
A vertical alignment type liquid crystal display element is formed by placing a vertical alignment (VA) mode liquid crystal cell between polarizers arranged in the crossed Nicols. The VA mode liquid crystal cell consists of a liquid crystal layer placed between two front and back glass substrates, and liquid crystal molecules in the liquid crystal layer are aligned approximately perpendicular to the substrates. When viewed from a perpendicular direction to the glass substrates, optical transmission of a background display region (no-voltage applied region) of the vertical alignment type liquid crystal display element is almost similar to that of the polarizers arranged in the crossed Nicols and very low. Therefore, it is relatively easy to realize high contrast display with the vertical alignment type liquid crystal display element.
The following techniques are well-known as alignment processes for uniformly pretilted liquid crystal molecules away from the perpendicular direction between inner surfaces of the front and back substrates of the VA mode liquid crystal cell: (i) a method wherein a uniformed alignment is realized by an effect of the surface shape obtained by performing oblique deposition of a metal oxide film such as SiOx as an alignment film from a direction tilted away from the perpendicular direction to the substrate to form a surface of the deposited film in a sawlike or jagged shape; (ii) a so-called photo alignment method wherein an organic alignment film material such as polyimide, etc. is formed on an inner surface of a substrate and thereafter ultraviolet ray is irradiated to a surface of the alignment film from a direction tilted away from the perpendicular direction (refer to Japanese Patent No. 2872628); and (iii) a method wherein a vertical alignment film having a specific surface free energy is formed on an inner surface of a substrate and thereafter treated by a rubbing process (refer to Japanese Laid-Open Patent No. 2005-234254). Those methods are mono-domain alignment methods wherein liquid crystal molecules can be aligned in one direction in a liquid crystal layer of a VA mode liquid crystal cell when no voltage is applied.
FIG. 8A is a schematic cross sectional view of a vertical alignment type liquid crystal display element according to the prior art, and FIGS. 8B to 8D are schematic plan views showing parts of electrode structures of the element.
As shown in FIG. 8A, the conventional vertical alignment type liquid crystal display element consists of a front substrate 11, a back substrate 12 and a vertical alignment liquid crystal layer 13 placed between both substrates 11 and 12.
The front substrate 11 consists of a front glass substrate 11a, a segment transparent electrode 11b formed on the front glass substrate 11a and a front vertical alignment film 11c formed on the front glass substrate 11a and the segment transparent electrode 11b. Similarly the back substrate 12 consists of a back glass substrate 12a, a common transparent electrode 12b formed on the back glass substrate 12a and a back vertical alignment film 12c formed on the back glass substrate 12a and the common transparent electrode 12b. The segment transparent electrode 11b and the common transparent electrode 12b are, for example, formed of indium tin oxide (ITO).
The front and the back vertical alignment films 11c and 12c are treated with the alignment process in one direction, for example, by the above-described alignment method (ii) or (iii). When a directional coordinate system in a plane parallel to the front and the back substrates 11 and 12 is defined by defining a far side direction perpendicular to the surface of the drawing sheet as a twelve o'clock direction (90 degrees direction), a right direction in the surface of the drawing sheet as a three o'clock direction (0 degree direction), a near side direction perpendicular to the surface of the drawing sheet as a six o'clock direction (270 degrees direction), and a left direction in the surface of the drawing sheet as a nine o'clock direction (90 degree direction), for example, the front vertical alignment film 11c is treated with the alignment process to align the liquid crystal molecules in the twelve o'clock direction, and the back vertical alignment film 12c is treated with the alignment process to align the liquid crystal molecules in the six o'clock direction.
A vertical alignment liquid crystal layer 13 is a liquid crystal layer whose liquid crystal molecules are almost vertically aligned and placed between the front vertical alignment film 11c of the front substrate 11 and the back vertical alignment film 12c of the back substrate 12. For example, the vertical alignment liquid crystal layer 13 is formed of liquid crystal material having negative dielectric anisotropy and has a mono-domain structure. For example, when no voltage is applied between the segment transparent electrode 11b and the common transparent electrode 12b (hereinafter called the no-voltage-applied state), the liquid crystal molecules in the liquid crystal layer 13 are aligned approximately perpendicular to the front and the back substrates 11 and 12 When a voltage larger than threshold voltage is applied between both electrodes 11b and 12b (hereinafter called the voltage-applied state), a large part of the liquid crystal molecules in the liquid crystal layer 13 are tilted toward an in-plane direction of the substrates 11 and 12 in the alignment process direction.
A spacer 14 keeps a gap, for example, between the front substrate 11 and the back substrate 12. Both substrates 11 and 12 are adhered too each other by a sealing part 15.
A front viewing angle compensator 16 and a front polarizer 18, in this order, are placed on the front glass substrate 11a on an opposite side of the liquid crystal layer 13. Similarly a back viewing angle compensator 17 and a back polarizer 19, in this order, are placed on the back glass substrate 12a on an opposite side of the liquid crystal layer 13. The front and the back polarizers 18 and 19 are, for example, placed in crossed Nicols arrangement and also in an arrangement wherein an absorption axis and an alignment direction (the six o'clock direction) of the liquid crystal molecules in the center of the thickness of the liquid crystal layer 13 in the no-voltage-applied state cross each other at almost 45 degrees in a plane in parallel to the substrate surface. For example, the front polarizer 18 is placed to make its absorption axis direct in a 45 to 225 degrees direction, and the back polarizer 19 is placed to make its absorption axis direct in a 135 to 315 degrees direction.
The front and the back viewing angle compensators 16 and 17 are viewing angle compensators having negative uniaxial or negative biaxial optical anisotropy. When in-plane slow axes exist in the front and the back viewing angle compensators 16 and 17, the front and the back viewing angle compensators 16 and 17 are arranged to make the slow axes and transmission axes of the adjacent polarizers 18 and 19 approximately parallel to each other.
In application of the liquid crystal display element, a back light is placed behind the back polarizer 19. Light emitted from the back light transmits through the back polarizer 19 and the back viewing angle compensator 17 and is input to the liquid crystal cell. The front polarizer 18 blocks the light input to and transmitted through the liquid crystal cell in any regions when a larger voltage than a threshold voltage is not applied between the segment transparent electrode 11b and the common transparent electrode 12b and in regions except a display regions (regions where both electrodes 11b and 12b are overlapped in a direction perpendicular to the substrates 11 and 12 and where display is performed) even when the voltage is applied. Therefore, a viewer viewing the liquid crystal display element from the outside of the front polarizer 18 sees a dark display state (black display). On the other hand, in the display region when the voltage larger than the threshold voltage is applied between both electrodes 11b and 12b, the light input to the liquid crystal cell transmits through the liquid crystal cell and the front polarizer 18. Therefore, a viewer viewing the liquid crystal display element from the outside of the front polarizer 18 sees a light display state (white display).
FIG. 8B and FIG. 8C are schematic plan views respectively showing a part of the segment transparent electrode 11b and a part of the common transparent electrode 12b. Moreover, FIG. 8D is a schematic plan view showing the parts of both electrodes 11b and 12b viewed from a direction perpendicular to the front substrate 11.
The liquid crystal display element shown in FIGS. 8A to 8D is a so-called segment liquid crystal display element which realizes a shape of a display region mainly by a shape of the segment electrode 11b. The display regions in an arbitrary shape can be demarcated by a shape of an electrode in the segment liquid crystal display element. FIGS. 8B to 8D show the electrodes 11b and 12b regions that form the display region which reads a word “AUTO”. Moreover, the segment liquid crystal display element is driven by a simple matrix drive such as the multiplex drive.
In order to display the word “AUTO”, the segment transparent electrode 11b is formed on the front glass substrate 11a in a shape shown in FIG. 8B. Moreover, the common transparent electrode 12b is formed on the back glass substrate 12a in a shape shown in FIG. 8C. The front substrate 11 and the back substrate 12 are arranged in approximately parallel to each other with making the surfaces where the electrodes 11b and 12b are formed face each other. Moreover, the front substrate 11 and the back substrate 12 are adhered to each other by positioning them as shown in FIG. 8D to form an overlapped region where both electrodes 11b and 12b overlap in a shape of the word “AUTO”. With a combination of the electrode structures shown in FIGS. 8B to 8D and the mono-domain vertical alignment liquid crystal layer, it becomes possible to realize a good dark display state in the region other than the display region.
It is known that the liquid crystal display element shown in FIGS. 8A to 8D has the polar angular direction observation angle wherein the light display state can not be observed from the six o'clock direction whereas the good light display state can be observed from the twelve o'clock direction. The twelve o'clock direction is called the optimal viewing direction while the six o'clock direction is called the anti viewing direction.